Cemented hard materials as sintered and composite material consist of at least two phases, namely a metallic binder phase and one or more hard material phases. Their various properties can be weighted by means of the respective proportion of the metallic and hard phases and the desired properties of the cemented hard material, e.g. strength, hardness, modulus of elasticity, etc., can be set in this way. The hard material phase usually consists of tungsten carbide but can, depending on the application of the cemented hard material tool, also comprise cubic carbides such as vanadium carbide, zirconium carbide, tantalum carbide or niobium carbide, their mixed carbides with one another or with tungsten carbide and also chromium carbide or molybdenum carbide. It is also possible to use nitrogen-containing cubic carbides (“carbonitrides”), for example in order to influence the phase ratios of the boundary zones during sintering. Typical binder contents in the case of cemented hard materials are in the range from 5 to 15% by weight, but in the case of specific applications they can also be lower at down to 3% and higher at up to 40% by weight.
In the case of classical cemented hard material, the metallic binder phase comprises predominantly cobalt. Due to the liquid-phase sintering and the dissolution and precipitation processes of the carbidic phase occurring during this, the metallic phase after sintering contains proportions of dissolved tungsten and carbon, often also Cr if, for example, chromium carbide is used as additive, and in the case of corrosion-resistant cemented hard materials also molybdenum. Very rarely, rhenium or ruthenium are also used as additive. The proportions of such metals which form cubic carbides are considerably lower in the binder because of the very low solubility.
In the sintered state, the metallic binder phase surrounds the hard material phase, forms a contiguous network and is therefore also referred to as “metallic binder” or as “binder”. It is of critical importance to the strength of the cemented hard material.
For the production of cemented hard material, cobalt metal powder is usually mixed and milled together with hard material powders in liquids such as water, alcohols or acetone in ball mills or attritors. Here, deforming stressing of the cobalt metal powder takes place. The liquid suspension obtained in this way is dried, the granular material or powder produced (“cemented hard material mixture”) is pressed to form pressed bodies and subsequently sintered with at least partial melting of the metallic binder, then, if appropriate, machined by grinding to final dimensions and/or provided with coatings.
Grinding operations require some engineering outlay since fine dusts which are harmful to health are produced or grinding sludges are produced and these represent a loss and their environmentally responsible handling incurs costs. It is therefore desirable to control the change in size of the pressed body during sintering in such a way that grinding operations become as superfluous as possible.
In powder metallurgy and in ceramics, the change in size of the pressed body during sintering is referred to as shrinkage. The linear shrinkage (S1) of a dimension is calculated from the change in the dimension caused by sintering divided by the original dimension of the pressed body. Typical values for this linear shrinkage in the cemented hard material industry range from 15 to 23%. This value is dependent on numerous parameters such as organic auxiliaries added (e.g. paraffin, low molecular weight polyethylenes or esters or amides of long-chain fatty acids as pressing aids, a film-forming agent for stabilizing granules after spray drying, e.g. polyethylene glycol or polyvinyl alcohol, or antioxidants such as hydroxylamine or ascorbic acid). These organic auxiliaries are also referred to as organic additives. Further parameters which influence the shrinkage and its isotropy are, for example, the particle size and size distribution of the hard material powders, the mixing and milling conditions and the geometry of the pressed body. The more fundamental reason is that these parameters and additives influence the compaction process during pressing of the cemented hard material mixture to form the pressed body. Furthermore, elemental carbon or refractory metal powder are used as further additives (inorganic additives) to control the carbon content during sintering and these can likewise influence shrinkage and its isotropy.
In the case of axially pressed bodies, which are standard in industry, anisotropies in the pressed density occur due to internal friction and friction at the walls during compaction and these anisotropies cannot be eliminated even by varying the parameters of the previous batch. These density anisotropies lead to different shrinkages in two or even three dimensions in space (anisotropic shrinkage) and thus to stresses or even to cracks in the sintered piece and therefore have to be minimized as far as possible. It is generally experienced that the lower the shrinkage, the better the densifiability during pressing, the shrinkage can be controlled better in process engineering terms within the desired tolerances and the anisotropy of shrinkage can be reduced. Combined with appropriate design of the pressing materials, sintered parts which have or are close to final dimensions can then be produced. In the case of sintered parts having the desired final dimensions, grinding operations are then superfluous.
In the case of axial pressing, experience shows that there is a difference in the shrinkage perpendicular to and parallel to the pressing direction. However, in the case of simple geometries, e.g. cubes or plates having a square area perpendicular to the pressing direction, there are no significant differences in the two directions perpendicular to the pressing direction, so that it is sufficient to determine the shrinkage in only one of the two directions perpendicular to the pressing direction.
EP 0 937 781 B1 describes how the undesirable anisotropy of the shrinkage in the production of cobalt-bonded cemented hard materials made of tungsten carbide having a particle size of less than 1 μm by uniaxial pressing can be influenced by means of the particle size of the cobalt metal powder used as binder. It is desirable to obtain a shrinkage which is absolutely identical in the pressing direction and perpendicular thereto (=isotropic shrinkage), which corresponds to a value for the parameter K of one. The further the value of K is below one, the more anisotropic the shrinkage. The value of K should be at least 0.988 in order to avoid after-machining by grinding operations. For cemented hard materials containing 20% of cobalt, a K value of 0.960 is reported.
The K value can be calculated from the observed shrinkages S (in %) according to the following formula, where the indices “s” indicate perpendicular to the pressing direction, “p” indicate parallel to the pressing direction:
  K  =                    (                  Ss          /          100                )            +      1                      (                  Sp          /          100                )            +      1      
The global shrinkage Sg in percent can be calculated from the pressed density and the sintered density according to the following formula:
  Sg  =      100    ⁢          (              1        -                              (                                          pressed                ⁢                                                                  ⁢                density                                            sintered                ⁢                                                                  ⁢                density                                      )                                1            /            3                              )      
The global shrinkage does not take account of any differences in the 3 dimensions and is to be regarded as a mean of the shrinkages in the three directions in space. It makes prognosis of the shrinkage on the basis of the pressed density possible.
Owing to the health hazards associated with the dust of tungsten carbide/cobalt composites, as occurs, for example, in the grinding of sintered cemented hard material, and the often poor availability of cobalt as coproduct of nickel or copper production, there is considerable interest in replacing cobalt as binder phase.
Nickel-based binders have already been used as potential replacement for cobalt-based metallic binders, e.g. for corrosion-resistant or nonmagnetic types of cemented hard material. However, due to the low hardness and the high ductility at relatively high temperatures, such types of cemented hard material cannot be used for the cutting machining of metals.
Iron- and cobalt-containing metallic binder systems are therefore the center of interest and are already commercially available. Either element powders such as cobalt, nickel or iron metal powders or prealloyed powders are usually used as starting materials in the mix-milling with the hard material powders. The prealloyed powders represent the composition of the FeCoNi proportion of the binder which is desired after sintering even beforehand as prealloyed powder.
EP-B-1007751 discloses cemented hard materials containing up to 36% of Fe for cemented hard material applications. Here, performance advantages over cobalt-bonded cemented hard materials are achieved, since the sintered cemented hard material has a stable face-centered cubic (fcc) binder phase, in contrast to a cobalt-bonded cemented hard material which although it has an fcc binder phase after sintering changes into the hexagonal phase which is more stable at relatively low temperatures during use. This phase transformation results in a change in the microstructure, which is also referred to as work hardening, and a poor fatigue behavior, which cannot occur in the case of a stable fcc binder phase.
EPA-1346074 describes a cobalt-free type of binder based on FeNi for coated cutting tools made of cemented hard material. Here, no work hardening can occur due to the stability of the fcc binder phase which prevails over a wide temperature range from room temperature to the sintering temperature. As a result of the absence of cobalt, it can be assumed that the high-temperature properties (hot hardness) of the ductile binder are not satisfactory for particular applications, e.g. turning of metal.
It has long been known from DE-U-29617040 and the thesis of Leo Prakash (TH Karlsruhe, 1979) that cemented hard material comprising binder phases based on FeCoNi which display a phase transformation with martensite formation resulting from cooling after sintering display particularly high hot hardnesses and also a generally relatively high wear resistance and better chemical corrosion resistance. Although the region in which martensite can occur can be estimated from the phase diagram of the ternary system Fe—Co—Ni, the dissolved content of tungsten, carbon or chromium in the metallic binder after sintering results in a shift in the two-phase region in the sintered cemented hard material since these elements stabilize the fcc lattice type. A metallic binder phase comprising about 70% of iron, 10% of cobalt and 20% of nickel, which is composed of two phases as a result of a martensitic transformation during cooling, has been found to be particularly wear-resistant for some cemented hard material applications (B. Wittman, W.-D. Schubert, B. Lux, Euro PM 2002, Lausanne).
From a metallurgical point of view, it is advantageous to use the FeCoNi proportion of the metallic binder phase in prealloyed form as powder, since the use of element powders (e.g. Fe, Co and Ni powders) is known to result in locally different temperature and composition positions of the melt eutectics Co—W—C and Ni—W—C and Fe—W—C and thus in premature local shrinkage, inhomogeneities in the sintered microstructure and mechanical stresses. Chemical equilibria are therefore superimposed on the sintering process.
EP-A-1079950 describes processes for producing prealloyed metal powders comprising the alloy system FeCoNi. Here, coprecipitated metal compounds or mixed oxides are reduced by means of hydrogen at temperatures in the range from 300° C. to 600° C. to give the metal powder. As an alternative, prealloyed metal powders can also be produced by other processes in which it is possible for the metal components to be mixed by diffusion, for example mixing and heating of oxides. If the equilibrium phase composition of these powders predetermined by the overall composition consists of two phases at room temperature, these powders often contain proportions of a precipitated ferritic phase (body-centered cubic, bcc) as a result of cooling after production, and the fcc proportion (face-centered cubic, fcc) still present can be entirely or partly metastable. The alloy powders can thus be supersaturated at room temperature in respect of the bcc components to be precipitated, and the precipitation of bcc components can be promoted by mechanical activation of the powders even at room temperature. Due to the known poor deformability of bcc phases and their presence in finely divided form due to the precipitation, the bcc-containing cemented hard material powders obtained after mix-milling and drying are difficult to press. The result is low green densities, high and anisotropic shrinkages and a greater dependence of the pressed density on the pressing pressure, compared to element metal powders. Despite the pronounced homogeneity, prealloyed FeCoNi powders which tend to form two phases have therefore not been able to become established as starting material for the production of cemented hard material for process engineering reasons. Since the tungsten carbide is not deformed during pressing and only the metallic binder phase ensures the necessary ductility during pressing, the above-mentioned problems become increasingly apparent at a reduced binder content. Cemented hard materials having a martensitic binder state, which require a prealloyed binder powder having very high iron contents and thus high bcc contents, and low binder contents such as 6% can therefore be produced only with great difficulty in process engineering terms.